Polyimides generally have excellent heat resistance, mechanical properties and electrical properties, compared with other general purpose resins or engineering plastics. Therefore, polyimides are widely used for various applications as molding materials, composite materials, electrical/electronic materials, optical materials, and the like.
For example, a circuit substrate, such as an HDD suspension substrate or a semiconductor package substrate, typically has a substrate and a patterned polyimide resin layer. The substrate is a metal substrate or a substrate having a circuit pattern; and when the difference in coefficient of linear thermal expansion (CTE) between the substrate and the polyimide resin layer becomes large, warpage of the circuit substrate is likely to occur by heat or the like generated in its usage environment. Such warped circuit substrate is incapable of maintaining precise mount and, thus, there is a demand for reduction of warpage of the circuit substrate caused by heat or the like.
In addition, the patterned polyimide layer is typically obtained by subjecting a photosensitive polyimide precursor composite layer to light exposure through a photomask having a pattern; and then subjecting the exposed layer to a developing treatment (etching) with an alkaline solution. Therefore, the photosensitive polyimide precursor composition layer needs transparency for sufficiently transmitting exposed light (in particular, i-line) and moderate solubility in an alkaline solution.
As a polyimide having high transparency and low coefficient of linear thermal expansion (CTE), a polyimide containing cyclohexanediamine (CHDA) as a diamine component is known (see PTLs 1, 2, 3, and 4). The polyimide containing cyclohexanediamine as a diamine component has low coefficient of linear thermal expansion (CTE) (see, e.g., Example 3 of PTL 1) and, by virtue of its alicyclic structure, has higher transparency than an aromatic polyimide. On the other hand, cyclohexanediamine is generally expensive, and thus cost reduction is achieved by combining with other diamines, such as norbornene diamine (NBDA) (see PTLs 2 and 5). PTL 5 discloses that a block copolymer of an amic acid oligomer containing a structure derived from cyclohexanediamine and an imide oligomer containing a structure derived from other diamines (e.g., norbornene diamine) has high transparency and low coefficient of linear thermal expansion (CTE).